Imaging media whereby a recorded latent image is developed by laminar separation are known. Among such imaging media, the provision of images which rely upon the generation of heat patterns is also known. Thermally imageable media are particularly advantageous inasmuch as they can be imaged without certain of the requirements attending the use of silver halide based media, such as darkroom processing and protection against ambient light. Moreover, the use of thermal imaging materials avoids the requirements of handling and disposing of silver-containing and other processing streams or effluent materials typically associated with the processing of silver halide based imaging materials.
Various methods and systems for preparing actinically generated symbols, patterns, or other images have been reported, said methods and systems involving laminar separation to effect development. Examples of these can be found in U.S. Pat. No. 4,169,731 (issued Oct. 2, 1979 to Noshiro et at.); in U.S. Pat. No. 4,581,308 (issued Apr. 8, 1986 to Moriya et al.); in U.S. Pat. No. 4,271,255 (issued Jun. 2, 1981 to Cho et al.); in U.S. Pat. No. 4,511,641 (issued Apr. 6, 1985 to Busman et al.); in U.S. Pat. No. 4,963,462 (issued Oct. 16, 1990 to Wilczak et al.); in U.S. Pat. No. 4,356,251 (issued Oct. 26, 1982 to Cohen et al.); in U.S. Pat. No. 4,489,153 (issued Dec. 18, 1984 to Ashcraft et al.); in U.S. Pat. No. 4,489,154 (issued Dec. 18, 1984 to Taylor, Jr.); in U.S. Pat. No. 4,123,578 (issued Oct. 31, 1978 to K. J. Perrington, et al.); in U.S. Pat. No. 4,157,412 (issued Jun. 5, 1979 to K. S. Deneau); and in International Patent Application No. PCT/US87/03249 of M. R. Etzel (published Jun. 16, 1988, as Intentional Publication Number WO 88/04237).
In the aforementioned International Patent Application No. PCT/US87/03249, there are described certain embodiments of a high resolution thermal imaging medium, which embodiments include a porous or particulate image-forming substance (e.g., a layer of pigment and binder) confined in a laminar structure between a pair of sheets. Upon laminar separation of the respective sheets, after laser exposure of portions or regions of the medium, a pair of complementary images is obtained. Among the laminar embodiments discussed in International Patent Application No. PCT/US87/03249 are those which include: a first sheet transparent to image-forming radiation and having at least a surface zone or layer of polymeric material which is heat-activatable upon subjection of the medium to brief and intense radiation; a layer of porous or particulate image-forming substance thereon; and a second sheet laminated and adhesively secured to the first sheet.
Upon exposure of regions or portions of the medium to brief and intense image-forming radiation, and conversion of absorbed energy to heat for activation of the heat-activatable polymeric material, corresponding regions or portions of the image-forming substance are caused m be more firmly attached or "locked" to the first sheet. Laminar separation of the sheet then effects fracturing of the image-forming layer in response to cohesive and/or adhesive failure. Abutting regions or portions of image-forming substance which are not subjected to such image-forming radiation are, upon separation of the first and second sheets, removed by the adhesive second sheet, for formation of an image complementary to the image on the first sheet.
The respective images obtained by separating the sheets of the exposed thermal imaging medium may exhibit substantially different characteristics. Apart from the imagewise complementary nature of these images and the relation that each may bear as a "positive" or "negative" of an original, the respective images may differ in character. Differences may depend upon the properties of the image-forming substance, on the presence of and nature of additional layer(s) in the medium, and upon the manner in which such layers fail adhesively or cohesively upon separation of the sheets. Either of the pair of images may, for reasons of informational content, aesthetics or otherwise, be desirably considered the principal image.
As described in the International Patent Application (and, e.g., U.S. Pat. Nos. 5,155,003 and 5,200,297), the image-forming layer may comprise image-forming substances (e.g., carbon black particles) and a hydrophilic binder (e.g., polyvinyl alcohol). According to one practice, carbon black particles are initially suspended in an inert liquid vehicle (typically, water) and the resulting suspension or dispersion uniformly spread over an underlying layer.
In the aforementioned International Patent Application, mention is made of particular embodiments comprising a release layer overlying the image-forming layer. As disclosed, the fracturable release layer comprises a microcrystalline wax. Subsequent patents, e.g., U.S. Pat. No. 5,155,003, issued to K. C. Chang on Oct. 13, 1993, and U.S. Pat. No. 5,200,297, issued to N. F. Kelly on Apr. 6, 1993, suggest the incorporation into a release layer of particulate materials (such as silica, day particles, and particles of polytetrafluoroethylene) and binding agents. Laminar separation of such medium will effect the fracturing of the image-forming layer and release layer in response to cohesive and/or adhesive failure, the failure being attributable to either of the two layers. It will be appreciated that, in general, adhesivity and cohesivity will oftentimes be affected by ambient environmental conditions.
While good results are obtained from the aforedescribed embodiments, a desire has developed to improve fracturing of the medium's fracturable layers and thereby enhance imaging performance. A desire has also developed to extend the performance of such media across a wider range of environmental conditions without departing from good imaging performance. 0f particular interest is the realization of such performance under varying environmental conditions, for example, under tropical (i.e., hot and humid) and arid (i.e., hot and dry) conditions, these conditions being foreseeably encountered when such media are transported, stored, and/or used during imaging and development. Advantage may be derived from reduced implementation of environmentally-insulating packaging, transportation, and storage, as well as by ameliorating the affects on imaging of environmental conditions.